X-ray backscatter imaging relies on scanning an object with a well collimated beam, often referred to as “pencil beam”. In the past these beams had also been widely used for X-ray transmission imaging, but today fan and cone beams in conjunction with pixelated detectors dominate transmission imaging.
There are two commonly used approaches for forming the collimated scanning beam. Both rely on a stationary X-ray source and a moving aperture. In both cases the radiation from a stationary X-ray source is first collimated into a fan beam by a stationary collimator. Then a moving part with an opening forms the scanning beam. This moving part is either a rotating disk with radial slits or a wheel with openings at the perimeter. The rotating disk covers the fan beam and the scanning beam is formed by the radiation emitted through the slits traversing the length of the fan beam opening. This approach is illustrated, e.g., in the 1973 U.S. Pat. No. 3,780,291 by Stein and Swift; see FIG. 1A. In the other approach a wheel with radial bores spinning around the X-ray source constitutes the moving part. If the source is placed at the center of the wheel the scanning beam is emitted in radial direction with the angular speed of the wheel.
Other approaches for forming a collimated beam from a stationary X-ray source have been proposed, for instance based on a rotating cylinder with a spiral groove as described by Annis in the 1996 U.S. Pat. No. 5,493,596.
System configurations with moving X-ray sources have been developed. The motion of the X-ray source is typically not formed by moving the X-ray tube but by moving (scanning) the electron beam along an extended anode. This produces a moving X-ray source point (focal spot of the electron beam) the location of which can be electronically controlled. A moving X-ray source point enables the formation of the scanning X-ray beam with a simple stationary aperture (pinhole) at some distance from the source point. As the X-ray source point is moved from one end of the scan path to the other the scanning X-ray beam emerging form the aperture spans an angular range. An embodiment of this concept is, e.g., part of the system described in the 1977 U.S. Pat. No. 4,045,672 by Watanabe, see also FIG. 1B.
As the X-ray beam covers the angular range the cross-sectional area of the beam varies as the cosine of the angle between the beam and the normal of the aperture plane. If the angular range is small the beam variation is limited and may be neglected. If, however, a large angular range is needed the effect becomes significant. For instance, for a 120° angular range an off-normal angle of 60° at the extremes leads to at least a 50% reduction in beam size and delivered flux, as the cosine of 60° is one half.
In reality the beam variation is even larger as the material with the pinhole has a finite thickness which leads to a further reduction in beam cross-sectional area with increasing angle. This problem becomes more serious for X-rays of higher energies which require thicker shielding material for the material with the pinhole.
To allow for thick shielding material and to avoid the angular variation it has been suggested to replace the pinhole with a rotating cylinder containing a bore perpendicular through the axis as described in the 2002 U.S. Pat. No. 6,356,620 by Rothschild and Grodzins, see also FIG. 1C. This cylinder would have to rotate synchronously with the scanning electron beam so that the moving X-ray source point is aligned with the bore at any time. This approach solves both of the problems with the simple pinhole design: It forms a beam of constant size independently of the beam angle and does not limit the thickness of the material forming the aperture. However, this active solution introduces significant cost and complexity in comparison to the passive pinhole. It also largely eliminates the great flexibility offered by the electronic control of the electron beam.